phrase (var. I do not believe it!)
an expression of utter disbelief at a truly unfathomable circumstance. Popularised by the grumpy old man Victor Meldrew (Richard Wilson) from the popularBBC television situation comedy series, One Foot in the Grave, written by David Renwick. Although "I don't believe it" is the catchphrase of the show, the first time Victor said it was in the last few minutes of "The Return of the Speckled Band" - the final episode of the first series. Profanities such as ‘bloody’ and the rather taboo ‘fucking’ are commonly inserted by the proletariat to create a heightened sense of shock but are rarely uttered by the more refined classes of middle England.

From the sitcom;
On discovering a lamppost had crashed through his bedroom window 'Victor Meldrew' exclaimed "I don't believe it!"

Colloquial use;
Dave: Look Barbara, malt loaf is on 'buy one get one free'
Barbara: I don't believe it!